Waterjet based processes are common in the field of micromachining. In particular, it is known to impinge a waterjet on a workpiece to cut, mill and turn the workpiece, for example. When waterjet methodologies are used to fabricate small parts, (e.g., less than one inch thick), and from thin, soft, and/or flexible materials, mechanical forces exerted by the high pressure waterjet transmit to the surface of the workpiece. These forces cause problematic deflections, vibrations, bending, and sometimes torque, on the workpiece. This undesired movement of the workpiece can result in poor dimensional and cutting precision,
Thus, the practice throughout the industry is to avoid waterjet processing in many cases where waterjet processes simply do not have the required capabilities to precisely process thin or flexible materials. Instead, other non-contact, process technologies such as laser-cutting, milling, turning, and the like are used. Further, many operators of waterjet processes characteristically run their processes at slower than desired speeds in attempts to overcome these force induced deflection problems. Otherwise, waterjet processes are preferred because they impart no heat to the workpiece, do not alter the chemical composition of the work surface, and are less costly.
In fluidjet processes such as waterjet -cutting, -milling or -turning, the material removal process that occurs can be described as a supersonic fluidic erosion process. It is the velocity of the stream as opposed to stream pressure that tears away microscopic pieces or grains of material from the workpiece. Pressure and velocity are therefore two distinct forms of fluid stream energy where velocity is the dominant parameter that correlates with the work that is done on the workpiece. When pure water is pressurized up to 60,000 pounds or more per square inch (psi) and forced through a tiny, pin-hole opening, it can generate a velocity that can cut a wide variety of materials including paper, plastic, metal, rubber and foam. When small amounts of abrasive particles, such as garnet, are mixed into the jet stream, the resulting “abrasive waterjet” can cut virtually any thickness of any hard material such as metal alloys, composites, ceramics, stone and glass.
Pure water that is pumped by a high pressure pump and flowing through narrow pipes can have sufficient energy to erode matter from a workpiece as a result of stream velocities enabled by a small gem based orifice. The gem based orifice is made from a hard jewel material, e.g. diamond, ruby, sapphire having a tiny thru-port therein, and the hole is supplied with fluid by plumbing tubing as is known in the art. The pressurized water passes through this tiny thru-port, thereby converting the pressure to an extremely high velocity. At approximately 40,000 psi the resulting stream that passes out of a typical gem based orifice is traveling at Mach 2. And at 60,000 psi the speed is over Mach 3. A diameter of a thru-port for a pure waterjet gem based orifice-ranges from 0.003 to 0.010 inch for typical cutting operations.
A gem based orifice (also known as a jewel orifice) with a single thru-port is the present design of nearly all known gem based waterjet orifices which in turn generate a single stream of fluid, with the opening size of the thru-port being the main factor determining the size of the resultant stream. The three most common types of gem based orifice materials include sapphire, ruby, and diamond. Each material has its own unique attributes. Sapphire is the most common gem based orifice material and is a man-made, single crystal jewel having a fairly good quality stream. A gem based sapphire orifice has a life, with good water quality, of approximately 50 to 100 cuffing hours. In abrasive waterjet applications, the sapphire's life is ½ that of pure waterjet applications. Sapphires typically cost between $15 and $30 each. Diamond has a considerably longer run life (800 to 2,000 hours) but is 10 to 20 times more costly. Diamond is especially useful where a 24 hour per day operation is required.
When cutting relatively thin materials (for example less than 1 inch thick and greater than about ⅛th inch thick), a conventional waterjet machine typically cuts parts having a tolerance ranging from ±0.003 to ±0.015 inch (0.07 to 0.4 mm). For very thin materials, for example less than ⅛ inch thick, this tolerance can increase substantially depending upon the material and can be 2 to 3 times as great or greater. However, the cutting speed must be reduced to obtain tolerances within this large range. For materials over 1 inch thick, known machines will produce parts having dimensional tolerances from about ±0.005 to 0.100 inch (0.12 to 2.5 mm). Again, very slow cutting speeds must be used to cut these thicker materials. Thus, when very tight tolerances (for example <10 microns) are required regardless of the workpiece thickness and specifically when tight tolerances and fast process speeds are desired, the waterjet process in general is challenged.
It is the inventors' discovery that a large part of the problem with using high velocity gem based orifices lies with the imbalance of forces exerted by a single waterjet upon the workpiece. The combination of (primarily) velocity and (secondarily) pressure exerted by a waterjet downwards (or sideways) upon the surface of a workpiece can result in a force vertical or normal to the surface that falls within the range of about 0.5 to >5.0 pounds. Because the waterjet is translated into the side of, for example a rotating workpiece such as a rotating rod or pipe, there can be a second force vector that is in this same force range but orthogonal to the initial force vector. This force (orthogonal to the first force vector) can be sufficient in magnitude to generate bending, deflection, and/or vibration in that plane of the workpiece. Unfortunately, both force vectors from a single waterjet stream can work in concert to dynamically move the workpiece away from the waterjet in a manner that varies with time and process conditions. The effect is that the forces unpredictably move the workpiece away from an ideal contact region of the waterjet and are particularly problematic to materials that can deflect easily such as thin or flexible materials, which otherwise would be ideal candidates for waterjet processing. In addition, instabilities can exist within the flowing fluid that can also impart vibration to many materials, particularly to thin materials. Sufficient support must therefore be provided to (usually the underside and sidewalls of) the workpiece such that the forces and flow instabilities do not cause unacceptable movement or vibration in the workpiece.
In the case of thin rod-shaped rotating workpieces, it was discovered that the additional level of mechanical support required to fully prevent deflection and vibration resulted in a high level of torque being transmitted to the rotating workpiece. This additional mechanical support caused other unwanted, i.e. twisting-mode, distortions and related vibrations within the workpiece. In order to address these problems, the exemplary embodiments herein provide a low and no drag means to provide support and therefore to prevent deflection of the workpiece. The concept shown and described herein utilizes balancing forces and force distribution applied against the workpiece, in particular using at least two fluid jet streams during dynamic contact with the workpiece.
Furthermore, a third, but related problem also exists. When the waterjet is cutting through the workpiece, the waterjet stream will often deflect or disturb the jet coherency causing a decrease in the cutting power of the stream. This problem is referred to as “Beam Deflection” or “Stream Lag” and results in increased taper, inside corner problems, sweeping out of arcs and slower overall process speeds. The exemplary embodiments herein can favorably resolve this problem as well.
The embodiments described herein overcome these and other problems of the art by enabling high precision, multi-orifice fluid jet based micromachining.